The present invention relates to a gas sensor.
Most gas sensors which comprise a semiconductor of oxide such as SnO.sub.2, ZnO, Fe.sub.2 O.sub.3 or In.sub.2 O.sub.3 and which are commercially available or under investigation for development at present require two kinds of electrodes, i.e., a sensor electrode and a heater, whether the sensor is of the thick layer type, the film type or the sintered chip type. Such sensors necessitate a complicated manufacturing process and are costly because they generally comprise a base plate, sensing and heating film electrodes formed on the front surface and the rear surface of the base plate respectively, and four lead wires. Another gas sensor is known which comprises an electrode of platinum film formed on a quartz tube and trimmed in a spiral form, a gas sensing layer of metal oxide semiconductor covering the entire electrode and a heater disposed within the quartz tube. This sensor also has the disadvantage of requiring four lead wires and two power supplies for the sensor electrode and the heater.
On the other hand, known composite sensors include one comprising two gas sensing units having a semiconductor adhered to a coiled resistor, i.e., a heating wire, a gas sensing activity, although a high sintering temperature of at least 1000.degree. C. is required to give a product of high strength. Consequently the sintered piece has exceedingly lower strength than usual ceramics, failing to have mechanical strength sufficient to withstand the impact or vibration to be usually exerted thereon during use. Various proposals have therefore been made to provide gas sensors of improved mechanical strength, while silica prepared from an organosilicon compound is presently used as a binder for commercial gas sensors. Silica sol or the like is also used for others which are in the stage of research and development. However, when such a binder is used for sintering, SiO.sub.2 or Al.sub.2 O.sub.3 remains in the product. Since the silica or alumina is hydrophilic, the gas sensitivity of the sensor obtained is dependent largely on humidity and varies with time when the sensor is exposed to a highly humid atmosphere.